What is a ring-shaped coral island called? | Reference.com What is a ring-shaped coral island called? A: Quick Answer A ring-shaped coral island is called an atoll. According to Encyclopedia Britannica, an atoll is made up of sections of coral reef that form a closed shape around a central lagoon. Full Answer According to Britannica.com, the ribbons of reef that make up an atoll need not always be ring-shaped or circular, but they always form a closed shape around a lagoon of around 160 feet in depth. The coral reef is mostly underwater, with small islands or longer masses of land on the rim of the reef. Some atolls are inhabited. Inhabited atolls can be found in Polynesia, Micronesia and the Maldives. Atoll | Definition of Atoll by Merriam-Webster See atoll defined for kids Did You Know? If you are lucky enough to sail south and west of Sri Lanka in the Indian Ocean, you'll find the Maldives, a group of about 1,200 coral islands and sandbanks that form the Republic of Maldives.

Many islands in that independent nation demonstrate the archetypal atoll, and geographers often use them to point out the characteristic features of such coral islands. Given how prevalent atolls are there, it isn't surprising that atoll comes from the name for that kind of island in Divehi, the official language of the Maldives. Origin and Etymology of atoll Divehi (Indo-Aryan language of the Maldive Islands) atolu First Known Use: 1625 How is an atoll formed? | Reference.com How is an atoll formed? A: Quick Answer Atolls, which are circular islands or archipelagos located around a central lagoon, are formed when the soft stone of a volcanic island is worn away, leaving behind only the coral reef that formed at its outskirts. Many Micronesian islands are atolls. Full Answer The formation of an atoll begins when an underwater volcano erupts, creating a volcanic mountain beneath the surface. When this mountain builds up and approaches the surface, its rough edges provide an ideal surface for coral to develop. Over time, a circular coral reef builds around the mountain. However, this kind of volcanic rock tends to be soft and porous.

The ocean gradually erodes it away, leaving behind a circular reef and a large lagoon. The chemistry of the new lagoon, however, is acidic and kills the coral on the inside. This coral is pounded into sand by water action, and the sand covers and insulates the still-living coral, creating a sandy atoll island over time. Hawaii has a number of atolls in its northern archipelago, many of which provide habitats for seals and endangered animals. The Micronesian nation of Tuvalu and the Maldives are also atolls. Larger settled atolls often actively work to build the protective sand cover on their islands, dredging the ocean for material to add. Atolls are a feared ocean hazard to boats, but they are also home to diverse colorful species and make ideal scuba diving locations.

23.3 worksheet - AK.doc - Geology with Faulstich/benson at Macomb Senior High School - StudyBlue Last Modified: 2014-05-16 File Size: 2 Views: 2 Geology 23.3 – The Ocean Basin Name________________________________ Per.____ Date______ Define the following terms and explain how & where they form: 1) abyssal plains • Abyssal plain - a large flat area on the ocean floor, composed of sediments that originated mostly on continents. (flattest surface feature on Earth) 2) abyssal hills • Abyssal hills - One of a series of small rolling hills on the ocean floor that occur next to continental margins and oceanic ridges. 3) deep-sea trenches • Long, narrow steep-sided troughs that run parallel to continental margins or volcanic island chains called island arcs at subduction zones (deepest parts of the oceans) • Sites of earthquakes & volcanic activity • Continental plate that overrides ocean plate has volcanic mountain chain like the Andes of S. America or Cascades of N.

America • Ocean plate that overrides another ocean plate has island arcs 4) island arcs • Island arcs – A volcanic island chain that forms along a deep-ocean trench. 5) deep-ocean vents • Geyser that erupts underwater, most are associated with mid-ocean ridges 6) mid-ocean ridges • A long chain of mountains with a central rift valley that is located along a divergent boundary on the ocean floor. • Magma rises where plates are pulling apart & cools to form a ridge of new sea floor 7) fracture zones – Fracture zone – an area of an ocean ridge that is broken by transform faults. 8) seamounts • Seamount - a cone-shaped undersea mountain of volcanic origin that rises high above the ocean floor. 9) guyots • Guyot- a flat-topped seamount. 10) coral atolls • Coral atoll - a ring-shaped coral island surrounding a central lagoon. – First a coral reef grows around a volcanic island and encircles it called a fringing reef. – Then the volcanic island sinks as ocean crust beneath it sinks and so do the corals, but new corals form on top of the old forming a barrier reef.

(more distance between island and reef) – Finally the volcanic island sinks below sea level leaving only the circular reef (now called an atoll) with a central lagoon. Advertisement ) NOAA National Ocean Service Education: Corals Corals How Do Coral Reefs Form? Corals usually develop into one of three characteristic structures: fringing reefs, barrier reefs or atolls. Click the image to see an animation. Coral reefs begin to form when free-swimming coral larvae attach to submerged rocks or other hard surfaces along the edges of islands or continents. As the corals grow and expand, reefs take on one of three major characteristic structures fringing, barrier or atoll. Fringing reefs, which are the most common, project seaward directly from the shore, forming borders along the shoreline and surrounding islands. Barrier reefs also border shorelines, but at a greater distance. They are separated from their adjacent land mass by a lagoon of open, often deep water. If a fringing reef forms around a volcanic island that subsides completely below sea level while the coral continues to grow upward, an atoll forms. Atolls are usually circular or oval, with a central lagoon.

Parts of the reef platform may emerge as one or more islands, and gaps in the reef provide access to the central lagoon (Lalli and Parsons, 1995; Levinton, 1995; Sumich, 1996). In addition to being some of the most beautiful and biologically diverse habitats in the ocean, barrier reefs and atolls also are some of the oldest. With growth rates of 0.3 to 2 centimeters per year for massive corals, and up to 10 centimeters per year for branching corals, it can take up to 10,000 years for a coral reef to form from a group of larvae (Barnes, 1987). Depending on their size, barrier reefs and atolls can take from 100,000 to 30,000,000 years to fully form. As coral reefs grow, they establish characteristic biogeographic patterns. Click the image for a larger view. All three reef typesfringing, barrier and atollshare similarities in their biogeographic profiles.

Bottom topography, depth, wave and current strength, light, temperature, and suspended sediments all act to create characteristic horizontal and vertical zones of corals, algae and other species. These zones vary according to the location and type of reef. The major divisions common to most reefs, as they move seaward from the shore, are the reef flat, reef crest or algal ridge, buttress zone, and seaward slope. Coral Reef - The shape of the land, Forces and changes, Spotlight on famous forms, For More Information Coral Reef Coral Reef Photo by: Mika Specta Coral reefs are not only spectacular marine environments, but they are one of the oldest ecosystems (community of plants and animals interacting with their environment) on Earth. They are created by colonies of organisms called coral polyps (pronounced PAH-lips). Among biological organisms, only humans have the ability to alter the surface of the planet more than these tiny marine creatures. Over thousands to millions of years, they may form massive structures of limestone that alter the shoreline of continents. Although coral reefs cover 0.2 percent of the total area of the oceans, they are critically important for a diversity of marine species.

They provide a habitat for at least 25 percent of all marine animals, including sponges, more than 4,000 different species of fish, anemones, sea stars, crabs and other crustaceans, and clams and other mollusks. Home to so many different species, coral reefs are often referred to as the "rainforests of the oceans." They also provide physical barriers to the force of strong waves, protecting beaches, lagoons, and other coastal features lying behind them. Yet they are highly vulnerable to pollution, an increase in water temperature, and damage from tourism and coastal development. By the early twenty-first century, these threats had already claimed more than one-quarter of the world's coral. The shape of the land A coral reef is a wave-resistant limestone structure produced by living organisms, found principally in shallow, tropical marine waters. Limestone is a type of rock composed primarily of the mineral calcite, which is a crystalline form of calcium carbonate (CaCO 3 ). Thousands of species inhabit coral reefs, but only a fraction produce the calcium carbonate that crystallizes into the limestone that forms the reef. Coral reefs are one of the oldest ecosystems on the planet.

They provide a habitat for more than 4,000 marine animals and are often referred to as the "rainforests of the oceans." PHOTOGRAPH REPRODUCED BY PERMISSION OF AP/WIDE WORLD PHOTOS . Coral polyps are the most important reef-building organism. The species that secrete calcium carbonate are known as hard coral polyps or, simply, stony corals. In the western Pacific Ocean, more than three hundred species of stony corals exist. Coral polyps are invertebrates (pronounced in-VER-tuh-brets), or animals without backbones. They are related to anemones and jellyfish. In fact, a coral polyp looks similar to an anemone: it is a jellylike sac attached at one end to its skeleton. The open end, the mouth, is fringed with six stinging tentacles (or a multiple of six), which the polyp extends at night to feed. The size of coral polyps varies greatly, depending on the species. They may be as small as 0.04 inch (0.1 centimeter) in diameter or as large as 8 inches (20 centimeters) in diameter.

Worldwide, coral reefs cover an estimated 110,000 square miles (284,900 square kilometers). Many stony corals grow best in clear, salty water with a temperature between 70°F and 85°F (21°C and 29°C). Because of this, most reefs are found in the Pacific Ocean, the Caribbean Sea, the Red Sea, the Arabian Sea, and the Indian Ocean between the tropic of Cancer and the tropic of Capricorn, two parallel lines of latitude lying one-quarter of the way from the equator to the North and South Poles, respectively. Reefs are also found around Florida and southern Japan because warm-water currents flow into these areas from the Tropics. The shape of a coral reef depends on the species of coral building it. Different types of coral produce different shapes. Some are pointed and hard; others are round and soft. They may be robust or delicate. Various shapes in reefs include fingerlike branches, flat branches, knobs or wedges, boulders or balls, mushroom caps, and tablelike structures. Coral reef: Words to Know Atoll: The high point of a coral reef that is almost always exposed at low tide.

Symbiosis: The close, long-term association between two organisms of different species, which may or may not be beneficial for both organisms. Zooxanthellae: Microscopic algae that live symbiotically within the cells of some marine invertebrates, especially coral. Most coral reefs are brightly colored, but that color does not come from the coral polyps themselves. In fact, the bodies of the tiny creatures are clear. The calcium carbonate they produce to form their external skeleton is white, much like the bones of a human skeleton. Reef colors come from the microscopic, single-celled algae called zooxanthellae (pronounced zoe-ah-zan-THEL-ee) that live in the tissue of the coral. Zooxanthellae and coral polyps have what is termed a symbiotic (pronounced sim-bee-AH-tik) relationship. Symbiosis is the close, long-term association between two organisms of different species, which may or may not be beneficial for both organisms. In the case of the zooxanthellae and coral polyps, the relationship benefits both.

The algae provide the coral polyps with more than 90 percent of their nutrients, and the coral polyps provide the algae with essential minerals and a protected habitat. Their unique relationship allows coral reefs to exist and grow year after year. Coral reefs are generally divided into three classes: fringing reefs, barrier reefs, and atolls (pronounced A-toles). The simplest and most common type of coral reef, fringing reefs, form close to the shoreline of islands and continents. A shallow lagoon, a quiet stretch of water separated from the open sea, may or may not lie between the shoreline and the fringing reef. Barrier reefs also form parallel to the shoreline of an island or a continent, but farther away than fringing reefs. In addition, they are larger and may stretch for great distances. Wide, deep lagoons often separate barrier reefs from the shoreline. Atolls are ring-shaped coral reefs that enclose or nearly enclose a deep lagoon. They are typically found around islands that have sunk beneath sea level in the deep ocean. These islands are often the tops of underwater volcanoes. On the lagoon side of atolls, sediment from broken coral may collect and partially fill the lagoon, allowing vegetation to grow.

Eventually, the entire lagoon may be filled in, forming an island that may become inhabited. Atolls such as this are common in the Indian and Pacific oceans. Forces and changes: Construction and destruction Geologists have discovered that ancient corals existed on Earth as long ago as 400 million years. Present-day stony corals evolved over the last 25 million years. Most established coral reefs are between 5,000 and 10,000 years old. Although they are the largest structures of biological origin on the planet and represent thousands of years of history, coral reefs are extremely delicate. Their formation takes place only under certain conditions. Corals reproduce two ways. They may do so sexually by releasing eggs and sperm. Once fertilized, the eggs produce multitudes of free-swimming larvae (pronounced LAR-vee; immature forms of the coral polyps) that ocean currents carry great distances. After settling on a suitable hard surface, the larvae secrete their own calcium carbonate cups and grow into mature corals, thus forming a new colony and a new reef. Corals may also reproduce asexually by budding or forming new coral polyps attached to themselves by thin sheets of tissue and skeletal material.

Through budding, a single coral polyp can develop over time into a massive coral head. The hard, cuplike skeleton that a coral secretes around itself from its lower portion consists of clusters of calcium carbonate, one of the most common minerals on Earth. In order to produce calcium carbonate, stony corals need the assistance of zooxanthellae, which live within cells in the lining of a coral polyp's gut. A polyp not only provides the zooxanthellae Like other atolls, Kayangel atoll in the western Pacific Ocean is a ring-shaped coral reef that encloses a deep lagoon. Atolls are common in the Indian and Pacific oceans. PHOTOGRAPH REPRODUCED BY PERMISSION OF PHOTO RESEARCHERS . with a protected environment but with carbon dioxide and nutrients that the polyp gives off as waste products. In return, the zooxanthellae use the carbon dioxide to provide the coral polyp with nutrients, including glucose (sugar) and amino acids, through photosynthesis (pronounced foetoe-SIN-thih-sis). Photosynthesis is the process by which plants use energy from sunlight to change water and carbon dioxide into sugars and starches.

The polyp uses the compounds supplied by the zooxanthellae to create proteins, fats, carbohydrates, and calcium carbonate. Sunlight and clear water Since zooxanthellae need sunlight in order to photosynthesize, corals flourish in waters less than 230 feet (70 meters) deep. Maximum growth rates occur when the coral is in water less than 60 feet (18 meters) deep. Corals may grow at deeper depths, sometimes up to 300 feet (91 meters), but their ability to produce calcium carbonate is greatly reduced. As a result, they grow poorly. The growth rate of corals is also reduced if the water is murky and sunlight is not able to penetrate. Water temperature and the amount of salt in the water also affect growth rate. Corals may grow in water slightly below 70°F (21°C) and slightly above 85°F (29°C), but that growth is very slow. Stony corals also prefer waters where the salt concentration ranges between 32 and 42 parts per thousand. The concentration of oxygen in the water must also remain high.

When optimum conditions are present, corals may grow as much as 1.7 inches (4.5 centimeters) per year. Different species grow at different rates. In general, the larger the coral, the slower the growth. Very large corals may grow only 0.2 to 0.8 inch (0.5 to 2 centimeters) per year. Branching corals may grow much faster, up to 8 inches (20 centimeters) per year. Because of their fast growth rate and their treelike structure, branching corals are prone to damage from strong storm waves, which may break off their branching limbs. Large, dense corals are much more stable and less prone to damage from wave action. While a coral polyp is alive, it periodically rises up in its cuplike skeleton and secretes more calcium carbonate to form a new base or floor. In this way, the polyp enlarges its skeleton, creating chambers underneath its base. When a polyp dies, its skeleton remains, becoming the foundation on which a new polyp attaches and builds its skeleton. Coral reefs are composed of layer upon layer of polyp skeletons, sometimes numbering in the billions.

Only the thin, top layer contains living coral polyps. Other creatures add to the complex structure of a coral reef. A type of red algae known as coralline algae contribute to the framework of reefs by secreting their own encrusting skeleton that helps cement loose sediment on the reef. Other organisms that contribute reef sediments include sponges, clams, and snails. These marine creatures live in the holes and crevices of the reef. When they die, their remains provide a foundation for new coral polyps. Reef structure and forms Coral reefs remain underwater except when water levels periodically decrease, such as at low tides. At this time, only the highest part of the coral reef, the reef crest, is exposed, washed over by waves. Coral polyps that exist and grow on this section of a reef must be able to tolerate the Sun's heat and wave action. In order to do so, they generally grow in plate-like, stubby branching or massive structures. The upper surface is often encrusted with coralline algae. Corals that exist on the reef seaward from the reef crest are diverse and show the greatest range of forms. In areas where wave energy is still high, massive corals are predominant.

As the reef angles deeper and deeper beneath the water's surface, delicate branching corals take over. The outermost seaward slope is called the fore reef. It may angle down to the sea bottom by as much as 30 degrees. The fore reef consists of limestone boulders, coral branches, and smaller sediments. It may have deep channels cut into it, forming fingerlike structures that extend seaward from the reef. These help stabilize the reef and cut down the force of incoming strong waves. Charles Darwin. . Darwin and Atoll Formation While traveling aboard the H.M.S. Beagle in the mid-1830s, English naturalist Charles Darwin (1809–1882) devised the modern theory of coral atoll formation. Although widely known for his theory of evolution by natural selection, Darwin made many contributions to the science of geology. His theory of coral reefs is his best-known. Darwin proposed that fringing reefs, barrier reefs, and atolls represented a series through geologic time. He held that the transition from fringing reef to barrier reef to atoll could result from the upward growth of coral on the edge of a gradually sinking volcano.

He believed that barrier reefs represented a middle stage between fringing reefs and atolls, and that the ringlike appearance of an atoll with a central lagoon resulted from the total submergence of the summit of a volcano. In the 1950s, scientists from the U.S. Geological Survey, an earth science research and information agency, undertook extensive drilling programs on atolls in the Pacific Ocean. Hundreds of feet down, they encountered volcanic rock, proving Darwin's theory that atolls are perched over ancient submerged volcanoes. The area lying landward of the reef crest is known as the back reef. Sand and other fine-grained sediment often inhibit reef growth in this area, so various other marine organisms dominate. However, scattered stubby, branching, or low knobby corals may develop in water as shallow as 3 feet (1 meter). Beyond the back reef, the water begins to deepen again, to as much as 100 feet (30 meters) or more, within the lagoon. Protected from the full force of waves, the lagoon floor is smooth and fine-grained. While fringing and barrier reefs are common on the submerged portions of continents, atolls are not.

Atolls formed in the deep ocean around submerged submarine volcanoes. They began to grow as a small fringing reef around the shoreline of a volcanic island. Once the volcano had become extinct, the sea floor beneath it may have begun to subside or sink under the weight of the volcano. As the island slowly submerged over millions of years, the corals continued to grow upward to the surface of the water, keeping pace with the rate the island was sinking. The sides of some atolls reach depths as great as 1,500 feet (457 meters). Eventually, as the island slipped beneath the ocean's surface, a ring of coral reefs remained, surrounding a central lagoon. Cold-water Corals Most well-known corals exist in tropical coral reefs, but reef-forming corals also exist in deep, cold water. Known as Lophelia, these corals lack zooxanthellae, so they do not depend on sunlight for survival. Instead, they feed by capturing food particles with their tentacles from the surrounding water. They are found at depths ranging from 230 to 3,280 feet (70 to 1,000 meters).

These types of corals grow slower than their warm-water relatives, averaging 0.04 inch (0.1 centimeter) per year. The largest known Lophelia coral reef is found off central Norway at a depth of about 1,312 feet (400 meters). It measures almost 9 miles (14 kilometers) in length and 2 miles (3 kilometers) in width. It stands almost 98 feet (30 meters) in height. Cold-water corals are found throughout the western Atlantic Ocean from Nova Scotia to Brazil. They are also found in the eastern Atlantic, the Mediterranean Sea, the Indian Ocean, and eastern Pacific Ocean. Like warm-water reefs, Lophelia coral reefs support very rich communities of fish, shrimps, and other invertebrates. Some atolls formed when sea levels rose, submerging the tops of islands. The rise and fall of the sea level over the past few million years has been caused by changes in the volume of water tied up in land glaciers and ice sheets during the ice ages. When ice sheets grew in the Northern Hemisphere, the sea level dropped and coral reefs such as atolls were stranded above the waterline.

Since present-day water levels have not risen to what they were before the last ice age, which ended approximately 10,000 years ago, the tops of many atolls have remained exposed. Reef damage While coral reefs may be damaged by natural forces such as storms and hurricanes, they suffer the severest damage from human activity. Reefs are often destroyed by collectors who use coral to create jewelry and by fisherman who use poison or dynamite to catch fish around coral reefs. Because corals need sunlight and sediment-free water to survive, water pollution poses a grave danger. Oil spills, the dumping of sewage wastes, and the runoff of soil and agricultural chemicals such as pesticides all threaten the delicately balanced ecosystem of coral reefs. Global warming, an increase in the world's temperatures, is the biggest threat facing coral reefs. It is thought to be caused, in part, by the burning of fossil fuels and the depletion of the ozone layer, both brought about through human activities. Scientists believe a warming of water temperature, by even just a few degrees, can cause coral polyps to expel the zooxanthellae living inside them.

This results in coral bleaching, which is the whitening of coral colonies due to the loss of the zooxanthellae. The end result is death of the coral. While pollution and changes in the salt content of the water can also bring about coral bleaching, warmer sea temperatures seem to be the biggest culprit. Natural occurrences, such as El Niño (the irregular periods during which the normally cold waters off the coast of Peru are made warmer by the arrival of warm waters from the equatorial region), have been blamed for some coral bleaching. However, these events are short-lived. Global warming is not. Spotlight on famous forms Great Barrier Reef, off the northeastern coast of Australia The Great Barrier Reef, situated off Queensland state in Australia's northeast, is the largest structure on the planet created by living organisms. Approximately 1,250 miles (2,011 kilometers) in length, the coral reef consists of more than 2,800 individual detached reefs, separated by deep channels. It is separated from the Australian shoreline by a shallow lagoon that varies from 10 to 100 miles (16 to 161 kilometers) in width.

At its widest, the reef measures 45 miles (72 kilometers) across. It covers roughly 80,000 square miles (207,200 square kilometers), an area approximately as large as the state of Kansas. The reef can be seen from space and was first mapped by Apollo 7 astronauts in 1968. Geological evidence shows the reef began growing more than 25 million years ago. Its age and size are due to the very stable geological setting of the area off the Australian continent and the favorable circulation of oceanic water. Winds in the area help stir the water, keeping it a relatively constant warm temperature regardless of depth. The Great Barrier Reef covers roughly 80,000 square miles, an area approximately as large as the state of Kansas. It is the largest structure created by living organisms. PHOTOGRAPH REPRODUCED BY PERMISSION OF THE CORBIS CORPORATION . The reef is home to millions of living creatures. More than 350 species of stony corals make up the reef.

Countless other organisms inhabit the area, including more than 1,500 species of fish, 4,000 species of mollusks, 200 species of birds, as well as varieties of dolphins, whales, green turtles, and dugongs or "seacows." The Great Barrier Reef is a major tourist destination, attracting more than 2 million tourists each year. To prevent the reef's destruction, the Australian government established the Great Barrier Marine Park in 1975. Encompassing most of the reef, the park is the world's largest protected marine area. Despite this protection, the reef is still endangered by rising sea temperatures and human activity. In 2003, the Australian government sought to ban commercial and recreational fishing from more than 30 percent of the reef. Kiritimati, Pacific Ocean With a total area of 222 square miles (575 square kilometers), Kiritimati is the largest coral atoll in the world. It encloses a large lagoon, which accounts for almost half of its area, and more than 100 lakes or ponds.

Lying 145 miles (233 kilometers) north of the equator, it is part of the Republic of Kiribati, composed of 33 islands scattered across 2,400 miles (3,860 kilometers) of the Pacific Ocean. In the local language, "ti" is pronounced "s," so Kiritimati is pronounced ki-RIS-mas. English explorer and navigator James Cook (1728–1779) landed on the atoll in 1777. Because the day of his arrival was Christmas Eve, Cook named the atoll Christmas Island (not to be confused with the Australian-administered Christmas Island that lies in the eastern Indian Ocean south of Java). More than a century later, England claimed the atoll as part of the Gilbert and Ellice Islands colony, extending its rule over it. In the late 1950s and early 1960s, England and the United States conducted nuclear tests on the atoll. In 1979, the Republic of Kiritabi was granted full independence.

Despite the fact that some areas of Kiritimati still remain barren as a result of nuclear tests, the atoll boasts much wildlife. It is particularly important as a seabird-nesting site: an estimated six million birds use or breed on the atoll. In 1975, Kiritimati was declared a wildlife sanctuary. Maldives, Indian Ocean The Maldives is an archipelago (pronounced ar-keh-PELL-ah-go; a group or chain of islands) of almost 1,200 coral islands in the Indian Ocean, located about 420 miles (675 kilometers) southwest of Sri Lanka. The archipelago is 511 miles (823 kilometers) long and 81 miles (130 kilometers) at its greatest width. The total area including land and sea is approximately 34,750 square miles (90,000 square kilometers). About 2 percent of this is land. Many of the islands are small, level, and low-lying, often no more than 6.5 feet (2 meters) above sea level.

Some are gradually washing away into the ocean, while others are in the process of formation and are constantly growing in size. The island of Malé, location of the capital city, is the most densely populated and developed. It is 1.2 miles (2 kilometers) long and just over 0.5 mile (0.8 kilometer) wide. The islands are formed from the growth of coral over a long-submerged volcanic mountain range. A protective fringing coral reef surrounds each island, some of which have freshwater lagoons. These are true coral islands: there is no trace of yellow or black coloring in the sandy coral beaches, as there is on other beaches around the world. Because the soil on the islands is completely coral-based, it is poor in nutrients and thick jungles do not grow. The coconut palm is the most common tree, and it grows densely on many of the islands. There are no hills, mountains, or rivers on any of the islands. The Maldives is an archipelago of almost 1,200 coral islands in the Indian Ocean. The islands are formed from the growth of coral over a long-submerged volcanic mountain range.

PHOTOGRAPH REPRODUCED BY PERMISSION OF THE CORBIS CORPORATION atoll - definition, etymology and usage, examples and related words atoll n atoll an island consisting of a circular coral reef surrounding a lagoon *** Webster's Revised Unabridged Dictionary n Atoll A coral island or islands, consisting of a belt of coral reef, partly submerged, surrounding a central lagoon or depression; a lagoon island. *** Century Dictionary and Cyclopedia n atoll A coral island, consisting of a strip or ring of coral surrounding a central lagoon. Such islands are very common in the Pacific ocean. They often present an exceedingly picturesque appearance, a comparatively narrow strip of coral rock thinly coated with soil, and covered with a vigorous growth of cocoanut-, pandanus-, and breadfruit-trees, inclosing a large still sheet of water, usually of considerable depth, and often well supplied with fish. The circle of coral is sometimes complete, showing no apparent communication between the inclosed lagoon and the surrounding sea; but generally it is interrupted, and presents one or more openings suitable for the passage of boats.

*** Chambers's Twentieth Century Dictionary n Atoll a-tol′ or at′ol a coral island consisting of a circular belt of coral enclosing a central lagoon. n Atoll a-tol′ or at′ol, a coral island consisting of a circular belt of coral enclosing a central lagoon. *** Hundreds and hundreds of thousands of square leagues of sea, thousands of islands, reefs, atolls. "The Blue Lagoon" by H. de Vere Stacpoole In the whole atoll not two stones remained one upon another. "South Sea Tales" by Jack London The structure of the rocky accumulation in the encircling reefs and in the atolls is essentially the same as in the fringing reef. "Autobiography and Selected Essays" by Thomas Henry Huxley It is, in fact, an elongated, almost linear atoll, with islands scattered along its sheltered margin. "Narrative Of The Voyage Of H.M.S. Rattlesnake, Commanded By The Late Captain Owen Stanley, R.N., F.R.S. Etc. During The Years 1846-1850. Including Discoveries And Surveys In New Guinea, The Louisiade Archipelago, Etc.

To Which Is Added The Account Of Mr. E.B. Kennedy's Expedition For The Exploration Of The Cape York Peninsula. By John Macgillivray, F.R.G.S. Naturalist To The Expedition. In Two Volumes. Volume 1." by John MacGillivray The vapor flies to the far rim of the Sea of Atolls. "White Shadows in the South Seas" by Frederick O'Brien The population of the wide-strewn Paumota atolls was estimated as about 10,000 in 1840. "Influences of Geographic Environment" by Ellen Churchill Semple A man who desires to become a shaman must keep strictly to a diet of white tortillas and atole for five years. "Unknown Mexico, Volume 1 (of 2)" by Carl Lumholtz From the coral atolls of the Fijis hastened six score volunteers. "Italy at War and the Allies in the West" by E. Alexander Powell It was an edifice to be built up on data, bit by bit, like an atoll.

"Foe-Farrell" by Arthur Thomas Quiller-Couch Still another variety of reef is known as the "atoll" or "lagoon" reef. "The Human Side of Animals" by Royal Dixon Almost all are formed of coral, and most of them are atolls. "From Pole to Pole" by Sven Anders Hedin And they stay quite a bit from shore out around the coral atolls. "The Boy With the U. S. Fisheries" by Francis Rolt-Wheeler Through these reefs there are passages of great depth, called atolls. "My First Voyage to Southern Seas" by W.H.G. Kingston He owned atolls so remote and tiny that his smallest schooners and ketches visited the solitary agents but once a year. "A Son Of The Sun" by Jack London Coral reefs may be arranged under three classes; namely, fringing reefs, barrier reefs, and atolls. "Wealth of the World's Waste Places and Oceania" by Jewett Castello Gilson Come out, and I'll enroll you as a member of the band once more; for this is the coral atoll for me.

"Aladdin & Co." by Herbert Quick Bennett for informing me of an atoll-formed reef in latitude 28 deg. "The Last Cruise of the Saginaw" by George H. Read Coral formations may be classed as fringing or shore reefs, barrier reefs and atolls. "Encyclopaedia Britannica, 11th Edition, Volume 7, Slice 4" by Various But in the centre of each atoll there is a lagoon from fifteen to forty-nine fathoms deep. "Principles of Geology" by Charles Lyell The inhabitants of the atolls live contentedly on coco-nuts and fish; they are expert fishermen, having little else to do. "The Belief in Immortality and the Worship of the Dead" by James George Frazer *** I think she knew no distant lands, all sun and glare and smells, And illy-ollying coolie gangs and chiming temple bells; No lumber, grain or nitrate ports from Yukon to Peru, No lone palm-girdled atolls in the false Pacific blue.

"Coasting Brigantine - Unnamed" by Cicely Fox Smith In news: The poisoning of thousands of Laysan Albatrosses at Midway Atoll National Wildlife Refuge, in the Pacific, has provoked a major cleanup of lead-based paint on the atoll. *** In science: Based on its evolution on the color-color plane (Hasinger & van der Klis 1989), and its spectral and temporal properties, Aql X-1 has been classiﬁed as an ‘atoll’ source (Reig et al. 2000). Evolution of Spectral States of Aql X-1 during the 2000 Outburst From a compilation of RXTE data of several transient atoll sources, Gierli´nski & Done (2002) and Muno et al. (2002) have shown that the atolls form a “Z” shaped track in the color-color diagram, similar to Z sources. Evolution of Spectral States of Aql X-1 during the 2000 Outburst They conclude that both atolls and Z sources move along their tracks on the color-color plane and do not jump between the tracks.

Evolution of Spectral States of Aql X-1 during the 2000 Outburst They also investigated the correlations between the characteristic frequencies and the X-ray colors of the source and concluded that it did not behave like an atoll source. A previous outburst in June 1998 was detected using archival RXTE/ASM data , suggesting a tentative recurrence time of ∼3.8 years. Observations of millisecond X-ray pulsars Studying the correlated spectral and timing variability, they concluded that the behavior of XTE J0929 –314 was consistent with that of an atoll source. Observations of millisecond X-ray pulsars This 4.3 hr orbital period makes it the widest binary system among the accretion-driven millisecond pulsars and also the one most similar to the general population of the low-luminosity neutron-star LMXBs (the atoll sources). Observations of millisecond X-ray pulsars From the correlations between the spectral and timing variability it was also conclude that the behavior of XTE J1814 –338 was consistent with that of an atoll source (; see also ).

Observations of millisecond X-ray pulsars Low magnetic-ﬁeld object types The low magnetic-ﬁeld neutron-star systems are subdivided into Z sources, atoll sources (Hasinger & van der Klis 1989) and what I shall call the ’weak LMXBs’. A review of rapid X-ray variability in X-ray binaries Atol l sources, many of which are X-ray burst sources, cover a much wider range in luminosities, from perhaps 0.001 LEdd (much lower in transients, but below this level timing becomes diﬃcult) all the way up to the range of the Z sources (e.g., Ford et al. 2000; Lx overlaps may well occur; distances are uncertain). A review of rapid X-ray variability in X-ray binaries Luminosities attained by Z sources, GX atoll sources, ordinary atoll sources and weak LMXBs, respectively, as well as by neutron-star and black-hole transients. A review of rapid X-ray variability in X-ray binaries States and luminosities attained by Z sources, GX atoll sources, ordinary atoll sources and weak LMXBs, respectively.

A review of rapid X-ray variability in X-ray binaries Note that not only the differences between Z and atoll sources need explanation, but also those among atoll sources. A review of rapid X-ray variability in X-ray binaries The relation between upper kHz QPO frequency and, left: HBO frequency in Z sources (scaled between sources by an inferred spin frequency), drawn line is for a quadratic relationship, dashed line is best power-law ﬁt, right: νh in atoll sources, line is power law with index 2.01. A review of rapid X-ray variability in X-ray binaries Wijnands & van der Klis (1999a; WK) noted that in atoll sources (including weak LMXBs) and black holes νb and νh are correlated over 3 orders of magnitude (Fig. 2.11b , with the hump at νh a factor 8 to 2 above the break, decreasing with νb .

A review of rapid X-ray variability in X-ray binaries Filled circles represent black-hole candidates, open circles Z sources, crosses atoll sources, triangles the millisecond pulsar SAX J1808.4−3658, pluses faint burst sources and squares Cir X-1. . A review of rapid X-ray variability in X-ray binaries ***

Question: What do you call a ring-shaped coral island surrounding a central lagoon?
Answer: Atoll